U.S. patent number 8,233,510 [Application Number 12/363,826] was granted by the patent office on 2012-07-31 for dual output laser source.
This patent grant is currently assigned to Infinera Corporation. Invention is credited to Peter W. Evans, Charles H. Joyner, Masaki Kato, Radhakrishnan L. Nagarajan.
United States Patent |
8,233,510 |
Joyner , et al. |
July 31, 2012 |
Dual output laser source
Abstract
A dual output laser source provided on a substrate outputs light
from a first and second output. A portion of the light generated by
the laser is supplied to a first modulator via the first output. A
second portion of the light generated by the laser is supplied to a
second modulator via the second output. The first modulator is
provided on the substrate and generates a first modulated signal.
The second modulator is also provided on the substrate and
generates a second modulated signal. Each output of the laser is
used to provide continuous wave light sources to components on
photonic integrated circuit.
Inventors: |
Joyner; Charles H. (Sunnyvale,
CA), Nagarajan; Radhakrishnan L. (Cupertino, CA), Evans;
Peter W. (Mountain House, CA), Kato; Masaki (Palo Alto,
CA) |
Assignee: |
Infinera Corporation
(Sunnyvale, CA)
|
Family
ID: |
40677449 |
Appl.
No.: |
12/363,826 |
Filed: |
February 2, 2009 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20090279576 A1 |
Nov 12, 2009 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
61028719 |
Feb 14, 2008 |
|
|
|
|
61030806 |
Feb 22, 2008 |
|
|
|
|
Current U.S.
Class: |
372/26; 372/96;
372/29.021; 398/184 |
Current CPC
Class: |
H04B
10/505 (20130101); H04B 10/532 (20130101); H04B
10/5561 (20130101); H01S 5/0085 (20130101); H01S
5/12 (20130101); H01S 5/4012 (20130101); H01S
5/0265 (20130101) |
Current International
Class: |
H01S
3/10 (20060101) |
Field of
Search: |
;372/26 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1742389 |
|
Oct 2010 |
|
EP |
|
WO2007/103410 |
|
Sep 2007 |
|
WO |
|
Other References
Griffin et al., "Integrated DQPSK transmitter for
dispersion--tolerant and dispersion-managed DWDM transmission",
Optical Fiber Communications Conference, 2003, pp. 770-771. cited
by other .
Serbay et al., "Comparison of six different RZ-DQPSK transmitter
set-ups regarding their tolerance towards fibre impairments in
8.times.40Gb/s WDM-systems" Advanced Modulation Formats, 2004,
EIII/LEOS Workshop, IEEE, pp. 9-10. cited by other .
Cole et al., "100GbE Optical LAN Technologies [Applicants &
Practice]" IEEE Communications Magazine, vol. 45, No. 12, Dec.
2007, pp. 12-19. cited by other .
Doerr et al., "Monolithic DQPSK Receiver in InP With Low
Polarization Sensitivity" IEEE Photonics Technology Letters, vol.
19, No. 21, Nov. 2007, pp. 1765-1767. cited by other.
|
Primary Examiner: Stultz; Jessica
Assistant Examiner: Niu; Xinning
Attorney, Agent or Firm: Duane Morris LLP Soltz; David
L.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to U.S. Provisional Application
No. 61/028,719 filed Feb. 14, 2008 and U.S. Provisional Application
No. 61/030,806 filed Feb. 22, 2008 both of which are herein
incorporated by reference in their entirety.
Claims
What is claimed is:
1. An optical circuit comprising: a semiconductor substrate; and a
laser provided on the semiconductor substrate, the laser having
first and second outputs, the first output supplying first light
and the second output supplying second light; a first modulator
being configured to modulate the first light to thereby supply a
first modulated optical signal carrying first data stream; a second
modulator being configured to modulate the second light to thereby
supply a second modulated optical signal carrying a second data
stream; a polarization rotator configured to rotate a polarization
of the first modulated optical signal, such that the first
modulated optical signal has a first polarization and the second
modulated optical signal has a second polarization that is
different than the first polarization; and a polarization beam
combiner having an output, the polarization beam combiner being
configured to receive the first and second modulated optical
signals and combine the first and second modulated optical signals
at the output of the polarization beam combiner.
2. An optical circuit in accordance with claim 1, further
comprising a variable optical attenuator (VOA) coupled to said
first output of the laser, said VOA configured to monitor the power
level of or vary an output level of the first light.
3. An optical circuit in accordance with claim 1, further
comprising a variable optical attenuator (VOA) coupled to said
first output of the laser, said VOA being configured to vary an
output level of the first light.
4. An optical circuit in accordance with claim 2, wherein said
laser includes an active layer, said VOA is further configured to
attenuate part of the first light reflected from said first output
of the laser back toward said active layer.
5. An optical circuit in accordance with claim 3, wherein the VOA
is a first VOA, the optical circuit further comprising a second VOA
coupled to said second output of the laser, said second VOA being
configured to monitor the power level of said second light.
6. An optical circuit in accordance with claim 3, wherein the VOA
is a first VOA, the optical circuit further comprising a second VOA
coupled to said second output of the laser, said second VOA being
configured to vary the output level of said second.
7. An optical circuit in accordance with claim 5 wherein said laser
includes an active layer, said second VOA is further configured to
attenuate part of the second light reflected from said second
output of the laser back toward said active layer.
8. An optical circuit in accordance with claim 1 further comprising
an optical waveguide coupled to said first output of the laser,
said optical waveguide comprising a light absorbing material
configured to absorb a portion of said first light.
9. An optical circuit in accordance with claim 8 further comprising
a variable optical attenuator (VOA) coupled to said optical
waveguide, said VOA being configured to monitor a power level of
the first light.
10. An optical circuit in accordance with claim 8, wherein the
optical waveguide is a first optical waveguide, the optical circuit
further comprising: a second optical waveguide coupled to said
second output of the laser, said second optical waveguide
comprising a light absorbing material configured to absorb a
portion of said second light.
11. An optical circuit in accordance with claim 10, wherein the VOA
is a first VOA, the optical circuit further comprising a second VOA
coupled to said second optical waveguide, said second VOA
configured to monitor a power level of the second light.
12. An optical circuit in accordance with claim 1 further
comprising: an optical waveguide coupled to said first output of
the laser, the optical waveguide receiving the first light, and an
optical tap configured to receive a portion of said first light
from said optical waveguide.
13. An optical circuit in accordance with claim 12 wherein said
optical waveguide includes a light absorbing material configured to
absorb a predetermined amount of said portion of said first
light.
14. An optical circuit in accordance with claim 12 further
comprising an optical power monitor coupled to said optical tap,
said power monitor configured to monitor a power level of the first
light.
15. An optical circuit in accordance with claim 1, wherein the
optical waveguide is a first optical waveguide and the optical tap
is a first optical tap, the optical circuit further comprising: a
second optical waveguide coupled to said second output of the
laser, and second optical tap configured to receive a portion of
said first light from said optical waveguide.
16. An optical circuit in accordance with claim 15 wherein said
second optical waveguide includes a light absorbing material
configured to absorb a predetermined amount of said portion of said
second light.
17. An optical circuit in accordance with claim 15 further
comprising an optical power monitor coupled to said second optical
tap, said power monitor configured to monitor a power level of the
second light.
18. An optical circuit, comprising: a laser provided, the laser
having a first output and a second output opposite the first
output, the laser being configured to generate light such that the
first output supplies a first portion of the light and the second
output supplies a second portion of the light; a first modulator
provided, the first modulator being configured to receive the first
portion of the light and generate a first modulated optical signal
carrying a first data stream; a second modulator provided, the
second modulator being configured to receive the second portion of
the light and generate a second modulated optical signal carrying a
second data stream; a polarization rotator configured to rotate a
polarization of the first modulated optical signal, such that the
first modulated optical signal has a first polarization and the
second modulated optical signal has a second polarization that is
different than the first polarization; and a polarization beam
combiner having an output, the polarization beam combiner being
configured to receive the first and second modulated optical
signals and combine the first and second modulated optical signals
at the output of the polarization beam combiner.
19. An optical circuit in accordance with claim 18 wherein said
laser is a distributed feedback laser.
20. An optical circuit in accordance with claim 18 wherein said
first modulator is a Mach-Zender modulator.
21. An optical circuit in accordance with claim 18 wherein said
second modulator is a Mach-Zender modulator.
22. An optical circuit in accordance with claim 18 wherein said
first portion of the light is a continuous wave signal.
23. An optical circuit in accordance with claim 18 wherein said
second portion of the light is a continuous wave signal.
24. An optical circuit in accordance with claim 23, further
comprising a variable optical attenuator (VOA) coupled to said
first output of the laser, said VOA being configured to monitor a
power level of said first portion of the light supplied from said
first output.
25. An optical circuit in accordance with claim 23, wherein said
VOA is further configured to attenuate part of the first portion of
the light reflected from said first output of the laser back toward
said laser.
26. An optical circuit in accordance with claim 18 further
comprising a variable optical attenuator (VOA) coupled to said
second output, said VOA being configured to monitor a power level
of said second portion of the light supplied from said second
output of the laser.
27. An optical circuit in accordance with claim 26, wherein said
VOA is further configured to attenuate part of the first portion of
the light reflected from said first output of the laser back toward
said laser.
28. An optical circuit, comprising: a substrate; a laser provided
on the substrate, the laser having a first output and a second
output opposite the first output, the laser being configured to
generate light such that the first output supplies a first portion
of the light at a first power level and the second output supplies
a second portion of the light; a first modulator provided on the
substrate, the first modulator having a first optical loss
characteristic and being configured to receive the first portion of
the light and generate a first modulated optical signal carrying a
first data stream; a second modulator provided on the substrate,
the second modulator having a second optical loss characteristic
and being configured to receive the second portion of the light and
generate a second modulated optical signal carrying second data
stream; a polarization rotator configured to rotate a polarization
of the first modulated optical signal, such that the first
modulated optical signal has a first polarization and the second
modulated optical signal has a second polarization that is
different than the first polarization; and a polarization beam
combiner having an output, the polarization beam combiner being
configured to receive the first and second modulated optical
signals and combine the first and second modulated optical signals
at the output of the polarization beam combiner, wherein the first
power level of the first portion of the light corresponds to the
first optical loss characteristic and the second power level of the
second portion of the light corresponds to the second optical loss
characteristic such that the first modulated optical signal has a
power level substantially equal to a power level of the second
modulated optical signal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
Embodiments of the invention relate to the field of optical
communication devices. More particularly, the present invention
relates to an optical circuit providing a dual output laser source
used for an optical communication system.
2. Discussion of Related Art
Wavelength division multiplexed (WDM) optical communication systems
are known in which multiple optical signals, each having a
different wavelength, are combined onto a single optical fiber.
Such systems typically include a laser light source associated with
each wavelength, a modulator configured to modulate the output of
the laser, and an optical combiner to combine each of the modulated
outputs. Conventionally, WDM systems have been constructed from
discrete components. For example, the lasers, modulators and
combiners have been packaged separately and mounted on a printed
circuit board. More recently, however, many WDM components
including transmitters, receivers and passive devices have been
integrated onto a single chip also referred to as a photonic
integrated circuit (PIC) chip.
Typically, these transmitters include a plurality of semiconductor
lasers which provide continuous wave (CW) light signals to an
optical circuit. Each of these lasers supply light at a particular
wavelength where each wavelength is associated with a respective
optical channel within a known WDM communications channel grid
(e.g. ITU). Each of a plurality of modulators receive the CW light
signals as well as respective data streams and output modulated
optical signals at each particular wavelength. One type of
semiconductor laser is a distributed feedback (DFB) laser which
employs a diffraction grating structure that couples both forward
and backward propagating coherent light produced in an active
region of a semiconductor substrate. Lasers are considered active
components because they require electrical power to function.
Additionally, passive components including splitters, combiners,
waveguides, etc., do not require power to operate, but do occupy
valuable space on a PIC chip. Thus, as the requirement to transmit
more data using smaller device sizes becomes increasingly
important, there is a need to produce transmitters, receivers and
the like using fewer discrete components while minimizing power
requirements all at reduced costs.
SUMMARY OF THE INVENTION
Exemplary embodiments of the present invention are directed to an
optical circuit employing a dual output laser source. In an
exemplary embodiment, an optical circuit includes a substrate
having a laser provided thereon. The laser has a first output and a
second output opposite the first output. The laser is configured to
generate light such that the first output supplies a first portion
of the light and the second output supplies a second portion of the
light. The circuit further includes a first modulator provided on
the substrate where the first modulator is configured to receive
the first portion of the light and generate a first modulated
optical signal. A second modulator is further provided on the
substrate and is configured to receive the second portion of the
light and generate a second modulated optical signal.
In another exemplary embodiment, an optical circuit is provided
which includes a semiconductor substrate and a distributed feedback
laser provided on the semiconductor substrate. The distributed
feedback laser includes an active layer provided on the
semiconductor substrate which is configured to produce light having
a wavelength. A diffraction grating is disposed either above or
below the active layer and is spaced a distance from the active
layer. The diffraction grating has a predetermined grating period
associated with the wavelength of light. A first output of the
laser is configured to output a first portion of the light having a
first power level. A second output of the laser is configured to
output a second portion of the light having a second power level,
wherein the first power level is substantially equal to the second
power level.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side view of an exemplary laser source in accordance
with an embodiment of the present invention.
FIG. 2 illustrates an exemplary dual output DFB laser in accordance
with an embodiment of the present invention.
FIG. 3 is an alternative embodiment of an exemplary dual output DFB
laser in accordance with the present invention.
FIG. 4 is a block diagram of an optical circuit in accordance with
an embodiment of the present invention.
FIG. 5 is a schematic illustration of an optical transmitter
circuit in accordance with an embodiment of the present
invention.
DESCRIPTION OF EMBODIMENTS
The present invention will now be described more fully hereinafter
with reference to the accompanying drawings, in which preferred
embodiments of the invention are shown. This invention, however,
may be embodied in many different forms and should not be construed
as limited to the embodiments set forth herein. Rather, these
embodiments are provided so that this disclosure will be thorough
and complete, and will fully convey the scope of the invention to
those skilled in the art. It will be understood that when an
element or component is referred to herein as being "connected" or
"coupled" to another element, it can be directly connected or
coupled to the other element or intervening elements may be present
therebetween. In contrast, when an element is referred to as being
"directly connected" or "directly coupled" to another element,
there are no intervening elements present. In the drawings, like
numbers refer to like elements throughout.
FIG. 1 is a side view of an exemplary laser source 10 having a
first output 20 and a second output 30 opposite the first output.
Laser source 10 is a distributed feedback (DFB) laser formed on
substrate 5 which may be, for example, n-type InP. Laser 10 may be
provided on a PIC chip. An active layer 12 is provided on substrate
5 which has a plurality of quantum wells and oscillates light
generated by the input of electric current. A diffraction (or
Bragg) grating 15 is disposed between a top layer 16 (comprising a
cladding layer) and passive light guiding layer 14. Alternatively,
grating 15 may be disposed below active layer 12. These various
layers extend between first output 20 and second output 30 on
substrate 5. Diffraction grating 15 has a periodic corrugation
which replaces cleaved end mirrors of a typical laser. The
"feedback" of the laser occurs when the stimulated photons
generated by active layer 12 are reflected back to the active layer
via grating 15. This reflection occurs along various points over
the grating and corresponding points along the active layer
implying its "distributed" feature. In addition, the grating
selectively reflects photons at a particular wavelength according
to the Bragg condition defined by 2.LAMBDA.n.sub.eff=.lamda. where
.LAMBDA. is the grating period and n.sub.eff=n sin .theta. (where n
is the refractive index of the medium). In this manner, laser 10
generates light having a single wavelength .lamda..
Because DFB laser 10 does not have any fixed mirrors at the
respective ends of the laser cavity, each end of the laser supplies
light having approximately the same output power. A first portion
of this light having wavelength .lamda. is output via first output
20 at a first power level. A second portion of this light having
wavelength .lamda. is output via second output 30 at a second power
level. The first and second power levels may be equal such that
each portion of the light generated by the DFB laser 10 is supplied
to a plurality of optical modulators as described below. In this
manner, laser 10 outputs light from both outputs 20 and 30 of the
laser cavity to supply a continuous wave (CW) light source to an
optical circuit. Previously the majority of light generated by
laser 10 would be directed for output through only one output with
light from the other output being terminated by a power-monitoring
diode. This power monitoring diode also served to eliminate stray
light that would otherwise provide unwanted feedback into the laser
cavity, thereby impairing the wavelength control and line-width of
the laser. By outputting light generated within the laser cavity
through both outputs 20 and 30, additional CW light sources and
associated optical components (e.g. splitters) used to direct
portions of the laser light to various components within the
optical circuit are obviated. In addition, the introduction of
associated signal loss, required power for multiple laser sources
and valuable chip real estate consumed by these additional CW light
sources and associated components is avoided.
FIG. 2 is a block diagram illustrating DFB laser 10 having a laser
cavity 11, first output 20 coupled to an optical waveguide 40 and
second output 30 coupled to an optical waveguide 50. Laser cavity
11 is defined by the layers described with reference to FIG. 1. A
variable optical attenuator (VOA) 45 is coupled to waveguide 40. A
second VOA 55 is coupled to waveguide 50. Laser 10 and VOAs 45, 55
may be provided on a PIC chip. Each of the VOAs 45 and 55 are used
to monitor the power level of the portion of light output from
first output 20 and second output 30 respectively. Optical
waveguides 40 and 50 may include a light absorbing material and in
addition serve to electrically isolate the laser 10 from the VOA's
45 and 50. The combination of the optical waveguide 40 with a light
absorbing material and VOA 45 decreases the back reflection of
light output from first output 20. Similarly, the combination of
the optical waveguide 50 with a light absorbing material and VOA 55
decreases the back reflection of light output from second output
30. In particular, the diffraction grating 15 and active layer 12
form a DFB laser cavity from which light is generated to each of
the first and second outputs 20 and 30 as noted above. However, if
a strong reflection results from the interface of either the first
output with waveguide 40 or second output 30 with waveguide 50, the
reflected light returns into the laser cavity 11. This may
potentially form an additional or secondary cavity with a Fabry
Perot spacing. This secondary cavity competes with the cavity
formed by the active layer 12 and the diffraction grating 15 in the
laser cavity 11 to determine at what frequency (or wavelength) the
DFB laser 10 will operate. This may cause "mode hoping" where the
oscillation wavelength of laser 10 changes discontinuously making
it difficult to control the output of light at a specified
wavelength .lamda. associated with the period of grating 15. Thus,
waveguide 40 is coupled to VOA power monitor 45 which reduces the
back reflection of the portion of light outputted from first output
20 of laser 10. Output waveguide 41 supplies the portion of light
outputted via first output 20 to additional optical circuit
components. The laser power P1 supplied to waveguide 41 is equal to
P.sub.T/2-PVOA.sub.1 where P.sub.T is the total output power of
laser 10 and PVOA.sub.1 is the power absorbed by VOA 45. Similarly,
waveguide 50 is disposed between VOA power monitor 55 and output 30
which reduces the back reflection of the portion of light outputted
from second output 30 of laser 10. VOA 55 is coupled to an output
waveguide 51 which supplies the portion of light outputted via
second output 30 to additional optical circuit components. The
laser power P2 supplied to waveguide 51 is equal to
P.sub.T/2-PVOA.sub.2 where P.sub.T is the total output power of
laser 10 and PVOA.sub.2 is the power absorbed by VOA 55. The amount
of light absorbed by VOA 45 and 55 is controlled by the applied
voltage to each VOA element. FIG. 3 illustrates an alternative
embodiment of dual output DFB laser 10 employing tap waveguides to
couple the light from each of the DFB outputs to a power monitor
and associated output waveguide while reducing the amount of light
lost at the respective outputs. The DFB laser and optical taps may
be provided on a PIC chip. Laser 10 includes an optical waveguide
40 coupled to first output 20 and a second optical waveguide 50
coupled to second output 30. A first optical tap 60 is aligned with
output waveguide 41 such that the light output from output 20 at
wavelength .lamda. is evanescently coupled to tap 60. A first power
monitor 65 is coupled to tap 60 for monitoring the power of the
portion of light output from output 20 of DFB 10. The evanescent
coupling reduces the loss associated with light coupled to power
monitor 65. A second optical tap 70 is aligned with output
waveguide 51 such that the light output from output 30 at
wavelength .lamda. is evanescently coupled to tap 70. A second
power monitor 75 is coupled to tap 70 for monitoring the power of
the portion of light output from output 30 of DFB 10. In this
manner, less light is lost from each of the outputs 20 and 30 to
the respective output waveguides 41 and 51 through the use of
evanescent taps 60 and 70.
FIG. 4 illustrates a dual output DFB laser 10 used to drive a
plurality of modulators all of which may be provided on a PIC chip.
DFB laser 10 supplies CW light to a first modulator 70 and a second
modulator 80. In particular, first output 20 of DFB laser 10
supplies a portion of CW light to input 71 of modulator 70. Second
(or back) output 30 of DFB laser 10 supplies a portion of CW light
to input 81 of modulator 80. First and second modulators 70, 80 may
be, for example, Mach Zehnder (MZ) modulators configured to receive
respective portions of light from DFB laser 10. A first data stream
140 is supplied to first driver circuit 104 which drives MZ
modulator 70. MZ modulator 70 outputs a modulated optical dat.sub.a
signal t.sub.o output .sub.72. A second data stream 142 is supplied
to sec.sub.ond driver circuit 116 which drives MZ modulator 80. MZ
modulator 80 outputs a modulated optical data signal to output 82.
The drive signals may be coded for either phase or amplitude based
modulation formats. In addition, the losses associated with the
paths in each modulator 70 and 80 may be considered when
determining the power levels of the light from outputs 20 and 30
respectively especially for signals in the TE and TM modes as they
propagate through the modulators. In particular, the power of the
first portion of CW light from output 20 may be greater or less
than the power of the second portion of CW light from output 30
depending on the loss characteristics associated with the
particular modulators 70 and 80. For example, if the output power
from each of the modulators 70 and 80 is desired to be equal, but
the path losses associated with modulator 70 are greater than the
losses associated with modulator 80, then the power of the CW light
from output 20 may be greater than the power of the CW light from
output 30. Similarly, if the path losses associated with modulator
80 are greater than the losses associated with modulator 70, then
the power of the CW light from output 30 may be greater than the
power of the CW light from output 20. The power from each output 20
and 30 of DFB 10 may be managed by, for example, VOA's 45 and 55
(shown in FIG. 2). In this manner, DFB 10 may be configured to
generate light such that the first output 20 supplies a first
portion of the CW light at a first poser level and the second
output 30 supplies a second portion of the CW light at a different
power level from the first portion such that when the difference in
losses in the paths associated with modulators 70 and 80 are
considered, the final output power from the modulators is
equal.
FIG. 5 is a schematic illustration of an alternative embodiment of
the present invention employing dual output DFBs as the CW light
source in an optical transmitter implemented on a photonic
integrated circuit. In particular, each of the transmitter portions
100 to 100-9 includes dual output DFBs 10 to 10-9 which each output
CW light at one of a plurality of wavelengths. The CW light
supplied from first output of each of the DFB laser sources 10 to
10-9 is supplied to MZ modulators 106 and 112 via branching units
111. The CW light supplied from second output of each of the DFB
laser sources 10 to 10-9 is supplied to MZ modulators 126 and 130
via branching units 113. MZ modulators 106 and 112 supply the
modulated optical signals to branching unit 115. MZ modulators 126
and 130 supply the modulated optical signals to branching unit 117.
Branching units 115 associated with each of the transmitters 100 to
100-9 supply a first output signal 405 to 405-9. Branching units
117 associated with each of the transmitters 100 to 100-9 supply a
second output signal 401 to 401-9. First output signals 405 to
405-9 are combined by first multiplexer 410 and supplied to first
waveguide 411 which is coupled between first multiplexer 410 and
polarization rotator 510. The multiplexed first output signals 405
to 405-9 are polarization rotated by rotator 510 and supplied to
PBC 414 via waveguide 420. Second output signals 401 to 401-9 are
combined by second multiplexer 412. Second waveguide 413 is coupled
to second multiplexer 412 and supplies the second output signals
401 to 401-9 directly to PBC 414 without being polarization
rotated. Each of multiplexers 410 and 412 may be, for example AWGs.
PBC 414 combines the multiplexed outputs received via waveguides
420 and 413 and supplies the output to waveguide 415. In this
manner, dual output DFBs 10 to 10-9 are utilized in each
transmitter portion 100 to 100-9 to provide CW signals modulated by
MZ modulators 106, 112, 126 and 130.
While the present invention has been disclosed with reference to
certain embodiments, numerous modifications, alterations and
changes to the described embodiments are possible without departing
from the sphere and scope of the present invention, as defined in
the appended claims. Accordingly, it is intended that the present
invention not be limited to the described embodiments, but that it
has the full scope defined by the language of the following claims,
and equivalents thereof.
* * * * *